Acoustic wave devices are used in acoustic wave filters, such as high-frequency filters in communication devices, such as mobile phones. FIG. 8A is a plan view of conventional acoustic wave device 70A disclosed in Patent Literature 1. Acoustic wave device 70A is an acoustic wave resonator. Acoustic wave device 70A includes a piezoelectric substrate, interdigital transducer (IDT) electrode 71, and reflector electrodes 72 and 73. Electrodes 71, 72, and 73 are formed on the piezoelectric substrate. IDT electrode 71 includes comb-shaped electrodes 71A and 71B facing each other. Comb-shaped electrode 71A includes interdigitated electrode fingers 71D, and comb-shaped electrode 71B includes interdigitated electrode fingers 71H. Interdigitated electrode fingers 71D and 71H are interdigitated with each other. Reflector electrodes 72 and 73 are arranged at both sides in the propagation direction of an acoustic wave excited by IDT electrode 71.
Comb-shaped electrode 71A includes bus bar 71C, interdigitated electrode fingers 71D, and dummy electrode fingers 71F. Electrode fingers 71D and 71F are electrically connected to bus bar 71C. Dummy electrode fingers 71F face interdigitated electrode fingers 71H of comb-shaped electrode 71B. Comb-shaped electrode 71B includes bus bar 71G, interdigitated electrode fingers 71H, and dummy electrode fingers 71J. Electrode fingers 71H and 71J are electrically connected to bus bar 71G. Dummy electrode fingers 71J face interdigitated electrode fingers 71D of comb-shaped electrode 71A.
Dummy electrode fingers 71F and 71J disposed in non-interdigitated regions 75A and 75C confine transverse mode spurious responses, which are unwanted resonant components in these regions.
FIG. 8B is a plan view of another conventional acoustic wave device 70B disclosed in Patent Literature 1. Acoustic wave device 70B is an acoustic wave resonator. In FIG. 8B, components identical to those of acoustic wave device 70A shown in FIG. 8A are denoted by the same reference numerals. Comb-shaped electrode 71A includes weighted parts 71K, which are formed at spaces between each interdigitated electrode finger 71D and each dummy electrode finger 71F in non-interdigitated region 75A. Comb-shaped electrode 71B includes weighted parts 71M, which are formed at spaces between each interdigitated electrode finger 71H and each dummy electrode finger 71J in non-interdigitated region 75C.
Gradually changing the size of weighted parts 71K and 71M along bus bars 71C and 71G can effectively suppress the transverse mode spurious responses confined by dummy electrode fingers 71F and 71J.
In acoustic wave device 70A, short circuits may occur between IDT electrode 71 and reflector electrode 72 or 73, causing property degradation of acoustic wave device 70A. In acoustic wave device 70B, more short circuits may occur between IDT electrode 71 and reflector electrode 72 or 73.
The following is a description of acoustic wave device 70B in which frequent short circuits may occur.
FIGS. 9A to 9H are sectional views showing production processes of acoustic wave device 70B. In FIG. 9A, electrode film 82 is formed by a method, such as deposition or sputtering, on an upper surface of substrate 81 made of a piezoelectric material, such as LiTaO3. In FIG. 9B, resist film 83 is formed on an upper surface of electrode film 82. In FIG. 9C, resist film 83 is processed into a desired shape by photolithography. In FIG. 9D, electrode film 82 is processed to have a desired shape to form IDT electrode 71, reflector electrodes 72 and 73, and other components by, for example, dry etching, and then resist film 83 is removed. In FIG. 9E, protective film 84, which is a thin film made of dielectric material, such as SiO2, is formed by a method, such as deposition or sputtering, so as to coat electrode film 82. In FIG. 9F, resist film 85 is formed on the surface of protective film 84. In FIG. 9G, resist film 85 is processed to have a desired shape by, for example, exposure and development. In FIG. 9H, unnecessary portions of protective film 84, such as pad 86 for the input and output of electrical signals, is removed by, for example, dry etching, and then resist film 85 is removed. Finally, the laminated body is diced into individual chips, each of which is then mounted on a package by die bonding or other methods, and are wire bonded. A lid is fixed by welding or other methods so as to hermetically seal the package, thereby providing acoustic wave device 70B.
FIGS. 10A to 10C are sectional views showing the production processes of acoustic wave device 70B from the exposure of resist film 83 to its removal in more detail than in FIGS. 9B to 9D, respectively. FIG. 10C is a sectional view taken along a line 10C-10C shown in FIG. 8B.
In FIG. 10A, resist film 83 is exposed through mask 91 to exposure light emitted from a light source, and then developed. As a result, resist film 83 remains only in unexposed portions. Mask 91 includes mask portion 91B for forming IDT electrode 71, and mask portions 91A and 91C for forming reflector electrodes 72 and 73, respectively.
In FIG. 10B, resist film 83 shown in FIG. 10A becomes resist film 83A after the exposure and development. Resist film 83A includes resist films 92A, 92B, and 92C. In this case, unwanted resist films 93 may remain between resist film 92B which becomes IDT electrode 71 and each of resist films 92A and 92C which become reflector electrodes 72 and 73, respectively. Upon being dry-etched through resist film 83A including unwanted resist films 93, as shown in FIG. 10B, electrode film 82 cannot be etched in a desired shape because unwanted electrode films 94 remain as shown in FIG. 10C. As a result, short circuits may occur between IDT electrode 71 and reflector electrode 72 or 73, causing the property degradation of acoustic wave device 70B.